Techniques have recently been attracting attention for printing, such as ink jet or screen printing, with a metallic ink on a flexible plastic substrate, followed by sintering, to form desired conductive patterns. The metallic ink used here is desired to sinter at lower temperatures below the heat resistance temperature of the plastic substrate. As such metallic inks for conductive patterning, silver- or copper-containing inks that are sintering at lower temperatures have widely been developed. Among them, copper has advantages over silver for its more excellent electrical characteristics and lower price.
As the inks for copper patterning, there are known inks in which copper carboxylate is used as the copper material (Non-patent Publications 1 and 2) and in which copper fine particles are dispersed (Patent Publications 1 and 2). Since copper is prone to oxidation, sintering of such inks is often performed in a reducing atmosphere, such as of hydrogen or formic acid, which causes safety problems in industrialization. In addition, the sintering temperature is as high as 250° C. or higher, and thus can be applied to only some types of plastic substrates.
In order to solve these problems, there are proposed inks achieving low-temperature sintering, such as the one disclosed in Non-patent Publication 3, which is composed of copper formate, amine, and a solvent. This ink is made into a copper pattern by sintering at 120 to 160° C. in nitrogen atmosphere. However, sintering of such conventional copper ink which contains an organic copper compound or copper fine particles as a copper material may result in shrinkage of the printed film, and the sintered pattern may be different from the desired, as-printed pattern.
On the other hand, it has been reported that materials containing organic components, which decompose and vaporize at lower temperatures, are useful in inks such as those on plastic substrates, which need to be sintered at lower temperatures. As such compounds, silver β-ketocarboxylate compounds are known, which are reported to be capable of silver patterning at lower temperatures (Patent Publication 3).
As a corresponding copper compound exhibiting similar decomposition mechanism, a copper β-ketocarboxylate compound may be envisaged. Copper β-ketocarboxylate compounds have been studied over a long time, and used as a polymerization catalyst or a silver reducing agent (Patent Publication 4, Non-patent Publications 4 and 5).
However, these compounds have never been used as a material for copper patterning, partly because copper is more prone to oxidation than silver, and conductive copper patterns cannot be obtained by techniques similar to those for the silver β-ketocarboxylate compounds.    Patent Publication 1: JP-2007-321215-A    Patent Publication 2: JP-2008-13466-A    Patent Publication 3: JP-2009-197133-A    Patent Publication 4: JP-2007-46162-A    Non-patent Publication 1: Current Applied Physics, 9, 2009, 157    Non-patent Publication 2: Jpn. J. Appl. Phys., 49, 2010, 86501    Non-patent Publication 3: Thin Solid Films, 519, 2011, 6530    Non-patent Publication 4: Journal of Inorganic and Nuclear Chemistry, 14, 1960, 161    Non-patent Publication 5: J. Am. Chem. Soc., 132(1), 2010, 28